There have so far been proposed various kinds of the methods for producing diamond-coated cutting tools having a diamond coating layer which is formed by the gas phase synthetic method. In addition to superiority in the wear resistance characteristic of a diamond film, the adhesive strength between a diamond coating film and a substrate is particularly important for using them as a cutting tool. In order to raise the adhesive strength between the substrate and the coat, there are a method in which the adhesion is allowed to be compatible with the abrasion resistance by varying conditions during synthesis (JP Patent Kokai JP-2-283697, 3-69595 and 4-223806), in addition to a method in which the adhesion is increased by synthesizing diamond of finer grains (JP Patent Kokai JP-A-62-235295 and 1-290591), and a method in which irregularities are formed on the surface of a substrate by surface treatment to provide an anchor effect between the substrate and a diamond coating layer, whereby the adhesive strength is improved (JP-A-1-162770, 1-246361 and 4-154969).